1. Field of the Invention
The present invention relates generally to semiconductor devices having collars disposed about the peripheries of the contact pads thereof and, more specifically, to the use of stereolithography to fabricate such collars around the contact pads prior to securing conductive structures to the contact pads. Particularly, the present invention pertains to collars disposed about the peripheries of the contact pads of a semiconductor device component for enhancing the reliability of conductive structures secured to the contact pads. The present invention also relates to semiconductor device components including such collars.
2. State of the Art
Some types of semiconductor devices, such as flip-chip type semiconductor dice, including ball grid array (BGA) packages and chip scale packages (CSPs), can be connected to higher level substrates by orienting these semiconductor devices face-down over the higher level substrate. The contact pads of such semiconductor devices are typically connected directly to corresponding contact pads of the higher level substrate by solder balls or other discrete conductive elements.
Examples of materials that are known in the art to be useful in connecting semiconductor devices face-down to higher level substrates include, but are not limited to, lead-tin (Pb/Sn) solder, tin-silver (Sn/Ag) solder, tin-silver-nickel (Sn/Ag/Ni) solder, copper, gold, and conductive polymers. For example, 95/5 type Pb/Sn solder bumps (i.e., solder having about 95% by weight lead and about 5% by weight tin) have been used in flip-chip, ball grid array, and chip-scale packaging type attachments.
When 95/5 type Pb/Sn solder bumps are employed as conductive structures to form a direct connection between a contact pad of a semiconductor device and a contact pad of a higher level substrate, a quantity of solder paste, such as 63/37 type Pb/Sn solder, can be applied to the contact pad of the higher level substrate to facilitate bonding of the solder bump thereto. As the 95/5 type Pb/Sn solder and the 63/37 type Pb/Sn solder are heated to bond the solder bump to a contact pad of the higher level substrate, the 95/5 type Pb/Sn solder, which has a higher melting temperature than the 63/37 type Pb/Sn solder, softens when the 63/37 type Pb/Sn solder is reflowed. When the 95/5 type Pb/Sn solder softens, the gravitational or compressive forces holding the semiconductor device in position over the higher level substrate can cause the softened 95/5 type Pb/Sn solder bump to flatten, pushing the solder laterally outward onto portions of the surface of the semiconductor device that surround the contact pad to which the solder bump is secured and, in the case of fine pitch or spacing of balls, into the solder of an adjacent ball.
Assemblies that include semiconductor devices connected face-down to higher level substrates using solder balls are subjected to thermal cycling during subsequent processing, burn-in, testing thereof, and in normal use. As these assemblies undergo thermal cycling, the solder balls thereof are also exposed to wide ranges of temperatures, causing the solder balls to expand when heated and contract when cooled. Repeated variations in temperatures can cause solder fatigue, which can reduce the strength of the solder balls, cause the solder balls to fail, and diminish the reliability of the solder balls. The high temperatures to which solder balls are exposed during burn-in and thermal cycling can also soften and alter the conformations of the conductive structures.
The use of other conductive structures, which have more desirable shapes, such as pillars, or columns, and mushroom-type shapes, and consume less conductive material than solder balls, to connect semiconductor devices face-down to higher level substrates has been limited since taller and thinner conductive structures may not retain their shapes upon being bonded to the contact pads of a higher level substrate or in thermal cycling of the semiconductor device assembly.
The likelihood that a solder ball will be damaged by thermal cycling is particularly high when the solder ball spreads over and contacts the surface of the semiconductor device or the higher level substrate. Flattened solder balls and solder balls that contact regions of the surface of a semiconductor device that surround the contact pads thereof are particularly susceptible to the types of damage that can be caused by thermal cycling of the semiconductor device.
In an attempt to increase the reliability with which solder balls connect semiconductor devices face-down to higher level substrates, resins have been applied to semiconductor devices to form collars around the bases of the solder balls protruding from the semiconductor devices. These resinous supports laterally contact the bases of the solder balls to enhance the reliability thereof. The resinous supports are applied to a semiconductor device after solder balls have been secured to the contact pads of the semiconductor device and before the semiconductor device is connected face-down to a higher level substrate. As those of skill in the art are aware, however, the shapes of solder balls can change when bonded to the contact pads of a substrate, particularly after reflow of the solder balls. If the shapes of the solder balls change, the solder balls can fail to maintain contact with the resinous supports, which could thereby fail to protect or enhance the reliability of the solder balls.
The use of solder balls in connecting a semiconductor device face-down to higher level substrates is also somewhat undesirable from the standpoint that, due to their generally spherical shapes, solder balls consume a great deal of area, or xe2x80x9creal estatexe2x80x9d, on a semiconductor device. Thus, solder balls can limit the spacing between the adjacent contact pads of a semiconductor device and, thus, the pitch of the contact pads on the semiconductor device.
Moreover, when solder balls are reflowed, a phenomenon referred to as xe2x80x9coutgassingxe2x80x9d occurs, which can damage a semiconductor device proximate to the solder balls.
The inventors are not aware of any art that discloses peripheral collars that may be disposed individually around the contact pads of a semiconductor device so as to, at least in part, define the shapes of conductive structures to be bonded to the contact pads or to facilitate bonding of a conductive structure to a bond pad without completely reflowing the material of the conductive structures. Moreover, the inventors are not aware of methods that can be used to fabricate collars around either bare contact pads or contact pads having conductive structures protruding therefrom.
In the past decade, a manufacturing technique termed xe2x80x9cstereolithographyxe2x80x9d, also known as xe2x80x9clayered manufacturingxe2x80x9d, has evolved to a degree where it is employed in many industries.
Essentially, stereolithography as conventionally practiced involves utilizing a computer to generate a three-dimensional (3D ) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3D computer-aided design (CAD) software. The model or simulation is mathematically separated or xe2x80x9cslicedxe2x80x9d into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object, and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and non-metallic materials. Regardless of the material employed to fabricate an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries, followed by selective consolidation or fixation of the material to at least a partially consolidated, or semi-solid, state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer of the object to be fabricated. The unconsolidated material employed to build an object may be supplied in particulate or liquid form, and the material itself may be consolidated or fixed, or a separate binder material may be employed to bond material particles to one another and to those of a previously-formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size, whereas when a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer that can be generated. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges.
An early application of stereolithography was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.
In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials, and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive for same, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques. It has also been recognized in some industries that a stereolithographic object or component may be formed or built around another, pre-existing object or component to create a larger product.
However, to the inventor""s knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other, pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. In particular, the inventor is not aware of the use of stereolithography to fabricate peripheral collars around the contact pads of semiconductor devices, such as flip-chip type semiconductor devices or ball grid array packages. Furthermore, conventional stereolithography apparatus and methods fail to address the difficulties of precisely locating and orienting a number of pre-existing components for stereolithographic application of material thereto without the use of mechanical alignment techniques or to otherwise assuring precise, repeatable placement of components.
The present invention includes a dielectric collar that surrounds the periphery of a contact pad of a semiconductor device, semiconductor device components including such collars, and methods for fabricating the collars. The present invention also includes forming conductive structures of desired configurations with the collars, as well as other methods for using the collars of the present invention.
A collar incorporating teachings of the present invention surrounds the periphery of a contact pad exposed at the surface of a semiconductor device component, such as a semiconductor die, a chip scale package substrate, or a carrier substrate. The collar protrudes from the surface of the semiconductor device component. If the collar is fabricated before a conductive structure is secured the contact pad, at least a portion of the surrounded contact pad is exposed through an aperture defined by the collar. The aperture of the collar may be configured to impart at least a base portion of a conductive structure to be bonded or otherwise secured to the contact pad with a desired shape and dimensions.
Conductive structures of any useful configuration can be used with or defined by the collar of the present invention. Exemplary configurations of conductive structures that can be used with or defined by the collar include, but are not limited to, balls, bumps, pillars or columns, mushroom shapes, or other shapes. These conductive structures can be fabricated from solders, metals, metal alloys, conductor filled epoxies, conductive epoxies, and other conductive materials that are suitable for use with semiconductor devices.
As the collar of the present invention facilitates the use of conductive structures having shapes other than that of a solder ball, alternatively shaped, thinner conductive structures can be spaced more closely, facilitating a decrease in the possible pitch of contact pads on a semiconductor device component. In addition, some alternatively configured conductive structures, such as pillars and mushrooms, require less material than balls.
Since the collar protrudes from the surface of the semiconductor device component, when a conductive structure is bonded or otherwise secured to the contact pad exposed through the collar, the collar laterally surrounds at least a portion of the conductive structure. Accordingly, when a conductive structure is formed on or secured to a contact pad, or during bonding of the conductive structure to the contact pad of another device or substrate, the contact pad collar of the present invention laterally contains at least a base portion of a conductive structure extending therethrough and prevents the material of the conductive structure from contacting and wetting portions of the surface of the semiconductor device component adjacent to the contact pad.
The collar is preferably configured to contact a conductive structure extending therethrough so as to laterally support and protect at least the contacted portion of the conductive structure during thermal cycling of the semiconductor device, such as in the repeated use thereof.
In addition, use of collars according to the present invention, which may be of substantial height or protrusiong from a substrate so as to encompass the conductive structures at or approaching their heights, may eliminate the need for an insulative underfill conventionally applied between a die and a higher level substrate.
Another significant advantage of the collars of the present invention is the containment of the conductive material of the conductive structures, in the manner of a dam, during connection of a semiconductor device face-down upon a higher level substrate, thus preventing contamination or wetting of the passivation layer surrounding the contact pads.
According to another aspect, the present invention includes a method for fabricating the collar. In a preferred embodiment of the method, a computer-controlled, 3-D CAD initiated process known as xe2x80x9cstereolithographyxe2x80x9d or xe2x80x9clayered manufacturingxe2x80x9d is used to fabricate the collar. When stereolithographic processes are employed, each collar is formed as either a single layer or a series of superimposed, contiguous, mutually adhered layers of material.
The stereolithographic method of fabricating the collars of the present invention preferably includes the use of a machine vision system to locate the semiconductor devices or other substrates on which the collars are to be fabricated, as well as the features or other components on or associated with the semiconductor devices or other substrates (e.g., solder bumps, contact pads, conductor traces, etc.). The use of a machine vision system directs the alignment of a stereolithography system with each semiconductor device or other substrate for material disposition purposes. Accordingly, the semiconductor devices or other substrates need not be precisely mechanically aligned with any component of the stereolithography system to practice the stereolithographic embodiment of the method of the present invention.
In a preferred embodiment, the collars to be fabricated upon or positioned upon and secured to a semiconductor device component in accordance with the invention are fabricated using precisely focused electromagnetic radiation in the form of an ultraviolet (UV) wavelength laser under control of a computer and responsive to input from a machine vision system, such as a pattern recognition system, to fix or cure selected regions of a layer of a liquid photopolymer material disposed on the semiconductor device or other substrate.
The collars of the present invention may be fabricated around the contact pads of the semiconductor device component either before or after conductive structures are bonded or otherwise secured to the contact pads, although it is preferred that the collars be fabricated before securing the conductive structures to the contact pads.
Other features and advantages of the present invention will become apparent to those of skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims.